1. Field of the Invention
The present invention relates to an optical pickup for reading information recorded on an optical recording medium, to a semiconductor laser device employed in such an optical pickup, and to a method of fabricating such a semiconductor laser device.
2. Description of the Prior Art
Optical pickup devices are used in optical memory devices, such as CD-ROM drives and MD drives, to read a signal from an optical disk, and some of them adopt semiconductor laser devices of a hologram laser type. A semiconductor laser device of a hologram laser type has a semiconductor laser element, a hologram element, and a signal-detecting light-receiving element built into a single package. The light emitted from the semiconductor laser element is reflected back from a disk used as an optical recording medium, and is then diffracted by the hologram element so as to be directed to the light-receiving element disposed away from the optical axis.
For example, Japanese Patent Application Laid-Open No. H6-5990 discloses a conventional semiconductor laser device of a hologram laser type, of which a perspective view is shown in FIG. 16. In the structure shown in FIG. 16, a semiconductor laser element (LD) and a signal-detecting light-receiving element or the like (not shown) are mounted on a stem 201, then a cap 202 is put on the stem 201 so as to cover the LD and the signal-detecting light-receiving element or the like, and then a hologram element 203 is fitted on the cap 202. supply of power to the elements disposed inside and the extraction of a signal therefrom are achieved by way of leads 208.
Moreover, in the package shown in FIG. 16, the stem 201 and the cap 202 are given the shapes of circles having upper and lower edges thereof cut off to slim down the package. In the package structured as shown in FIG. 16, it is necessary to form, in the stem 201, holes through which to put the leads 208, then put the leads 208 one by one through the holes, and then seal the holes with an insulating material. This requires a complicated fabrication method and high costs.
In recent years, to reduce the costs of optical pickup devices, attention has been paid to semiconductor laser devices fabricated with inexpensive packages made of resin and with leads formed in the form of a lead frame. For example, Japanese Patent Application Laid-Open No. H6-203403 discloses such a semiconductor laser device, of which the structure is shown in FIGS. 17A and 17B. FIG. 17A is a top view, and FIG. 17B is a side sectional view. In the structure shown in FIG. 17, a resin frame 302 having cylindrical portions 301 is used as a package, a plurality of leads 303 are disposed through each of two opposite sides of the resin frame 302 so as to run from outside to inside the package, and a hologram element 304 is fitted on the resin frame 302 so as to be located between the two cylindrical portions 301.
Now, a fabrication method applicable to the structure shown in FIG. 17 will be described with reference to FIG. 18, taking up as an example a semiconductor laser device having 12 leads in total, i.e. six leads through each of opposite sides thereof First, by stamping or etching, a strip-shaped sheet metal (hoop member) is formed into a lead frame 404 having island plates 401, on which to mount semiconductor elements, and leads 402. At this point, the island plates 401 and the leads 402 constituting one group and those constituting the adjacent groups are connected together by frame portions 403 in the form of continuous hoops (FIG. 18(a)).
Next, resin is molded on the lead frame 404 to form resin frames 405 that hold the leads 402 and the island plates 401 (FIG. 18(b)). After resin molding, individual packages 406 are cut apart (separated) from one another by cutting the lead frame 404 between the leads 402 and the frame portions 403.
With this structure, as opposed to the structure shown in FIG. 16 where it is necessary to put lead pins one by one through holes and then seal the holes with glass, it is possible to fabricate the semiconductor laser device through a series of processes suitable for mass production, specifically, stamping, molding, and cutting performed in this order. This helps greatly reduce costs.
Thereafter, as shown in FIG. 18(c), on the island plate 401 of each package 406, a submount 412 having an LD 407, a mirror 408, and a signal-detecting light-receiving element 409 mounted thereon is mounted, and then these are interconnected with Au wires 410 by wire bonding. Next, the LD. 407 is energized and left emitting light for several hours at a high temperature to screen out defective LDs, then its light emission characteristics are evaluated, and then a hologram element 411 is fitted. Here, the hologram element 411 serves also as the cap 202 in the package shown in FIG. 16. This helps reduce the number of components needed and thereby reduce costs. After the hologram element 411 is fitted, the semiconductor laser device is actually energized to inspect the signal obtained through light reception from a reference disk. This is to evaluate the electrical and optical characteristics of the semiconductor laser device and thereby pick out only acceptable ones. Now, the fabrication of the semiconductor laser device is complete.
In the example described above, the LD 407 emits light through a side end thereof, and therefore the mirror 408 is used to turn the optical axis by 90°. However, in a case where the LD 407 emits light through the top surface thereof, it is not necessary to use the mirror 408. In the example described above, the LD 407 is mounted directly on the island plate 401. However, a submount incorporating a monitoring photodiode for controlling the light output may be interposed therebetween.
In the semiconductor laser device described above with reference to FIGS. 17A, 17B, and 18, its optical axis is aligned substantially with the center of its external package. This is to ease the alignment of the optical axis when the semiconductor laser device is fitted to a pickup. Thus, the LD 407 and the mirror 408 are mounted at or near the center of the island plate of the package. On the other hand, the signal-detecting light-receiving element 409, which receives a signal beam separated from the optical axis by the hologram element 411, is disposed away from the optical axis. In the following description, the reference numerals used in FIG. 18 will be sticked to.
In this way, the LD 407 and the mirror 408 are located in the center of the island plate 401, and the signal-detecting light-receiving element 409 is located on one side of them. Here, the signal-detecting light-receiving element 409 incorporates a multisegment photodiode and, in some cases, even a preamplifier, and thus requires may signal terminals. Accordingly, many wires need to be connected to the light-receiving element, leading to a lopsided wire layout inside the package.
Moreover, as an increased number of wires are used, an accordingly increased number of leads need to be provided. Since more leads are required in that side of the package where the signal-detecting light-receiving element 409 is disposed, more leads are left unused in the opposite side.
If one dares to lay wires from one side of the package to leads in the opposite side, the wires run above the laser element (LD) 407 and the mirror 408, and thus intercept the beam.
How this looks is shown in FIGS. 19A and 19B, which show the conventional structure shown in FIG. 18 with wires additionally laid. FIG. 19A is a top view without the hologram element 411 fitted and FIG. 19B is a side view with the hologram element 411 fitted.
In the package shown in FIGS. 19A and 19B, there are provided 12 lead pins (402a to 402l) in total, i.e. six lead pins through each of two opposite sides thereof Of these lead pins provided, three 402a, 402f, and 402g are integral with the island plate 401. Thus, ten independent lead pins in total are provided. On the other hand, the number of lead pins required for wiring is as follows: the signal-detecting light-receiving element 409 requires eight, and the LD 407 and the submount 412 require three. Of these lead pins required, one used as ground (GND) by the signal-detecting light-receiving element 409 and one used as ground by the LD 407 can be made common. Thus, ten independent lead pins in total are required. Since the signal-detecting light-receiving element 409 requires more wires, two wires 410 need to be laid therefrom to the leads in the left, i.e., far, side, with the wires 410 running above the mirror 408.
This can be avoided by increasing the number of leads or securing a space, specifically by providing an extra lead or securing an extra space between the leads 402c and 402d and another between the leads 402i and 402j shown in FIG. 19. This permits the wires that otherwise need to be laid from the signal-detecting light-receiving element 409 to leads in the far side to be connected to leads in the near side, and thus helps prevent the wires from running above the mirror 408.
In a package of a lead frame type, increasing the number of leads results in increasing the thickness of the package and thus the thickness of the optical pickup that employs it. Moreover, increasing the size of the package results in increasing the costs of the metal and resin materials thereof and thus the overall costs thereof In particular, the hologram element 411 is expensive, and therefore it is desirable to make it as small as possible in order to minimize costs. However, in the structure shown in FIGS. 17A, 17B, and 18, since the hologram element 411 serves also as a cap, increasing the size of the package results in making its peripheral portions accordingly and unduly large. This leads to higher costs.
To slim down the package, wiring needs to be designed to leave as few leads as possible unused.
When the lead frame is still in the form of continuous hoops, to permit the island plates to be held connected to the frame portions, each island plate needs to be formed integral with at least one lead pin. However, when the lead frame in the form of continuous hoops is wound up or set on equipment, if the island plates are held by only one lead pin each, those leads may bend, causing the island plates to be displaced relative to the frame portions. For this reason, in practice, to hold the island plates and the frame portions stably, each island plate needs to be held at three points, specifically by three lead pints, i.e., one lead pin on either side plus another on one side. Thus, while one of these three lead pins is effectively used as being kept at the same potential as the island plate, the other two cannot be used independently for the wiring of semiconductor elements.
Before being cut out of the lead frame, the individual packages are arranged neatly. When cut apart, however, the packages need to be arranged in a tray for transfer, and bends in their leads need to be corrected.
On the other hand, when the lead frame is still in the form of continuous hoops, all leads are short-circuited. This makes it impossible to energize the LD and the signal-detecting light-receiving element for testing purposes.
In a case where the package is formed by molding resin, in particular thermoplastic resin, on the lead frame, application of heat thereto may soften the resin and thus cause deformation in the resin frame. Thus, wire bonding needs to be performed with supersonic waves alone without application of heat. This makes it difficult to obtain sufficient wire bonding strength, and increases faults in wire bonding.
In general, the reflecting member for deflecting the light emitted from the semiconductor laser element needs to be positioned with high accuracy. However, the accuracy with which it is actually mounted depends on the precision of die-bonding equipment. This makes it difficult to adjust the position of the reflecting member.
The light-receiving element is disposed on the same plane as the semiconductor laser element or the submount having the semiconductor laser element mounted thereon. This allows part of the light emitted from the semiconductor laser element to stray onto the light-receiving surface of the light-receiving element., and thereby degrades the S/N characteristic of the signal obtained through light reception.
The semiconductor laser element, as it emits light, generates heat. Since high temperature degrades the quality of the semiconductor laser element itself, it is necessary to efficiently reject heat to outside the package. In a package of a resin molding type, however, the back surface of the island plate, on which the semiconductor laser element is mounted, is covered by resin. This leads to poor heat rejection through the back surface of the package.
In addition to these problems, producing a pickup by using the conventional semiconductor laser device described above requires complicated position adjustment including the alignment of the optical axis.